This invention relates in general to the manufacture of color television picture tubes and in particular to an apparatus for suppressing erosion of the electron emissive coating on the cathode of an electron gun during manufacture of the tube. Conventionally, an electron gun used in a color television picture tube includes an electron beam source and an electron beam focus lens. The electron beam source typically comprises a heated cathode element and associated electrodes which collect electrons emitted by the cathode element and form them into a beam cross-over. The electron beam focus lens shapes the stream of electrons emitted by the cathode and focuses the beam cross-over on the screen of the tube. The electron beam focus lens typically comprises electrodes at varying potentials. The forward element is the focus lens anode and typically takes the form of a cup called a "convergence" or "shield" cup.
An electron gun for use in a color television picture tube generally comprises three guns, one each for exciting red, blue and green phosphor elements on the screen of the tube. Each of the electrodes and the shield cup in the gun have three apertures, one for each of the three cathodes which emit the streams of electrons. The apertures are generally circular and the apertures for each beam lie on a common line, that is they are coaxial. The apertures in the electron gun form beam passageways.
An electron gun which is used in a black and white television picture tube has only one aperture in each of the electrodes and convergence cup or shield cup. Thus there is only one stream of electrons emitted by the gun. Like the color television picture tube gun the apertures in the electrodes of the black and white gun also are coaxial.
In the manufacture of color televison pictue tubes or black and white picture tubes, after the tube is assembled, most of the gas, usually air, which is inside the tube must be evacuated. Conventionally, this is done by attaching a vacuum pump to a tubulator which is located at the rear of the neck of the tube. As the tube is evacuated, all of the gas which is drawn from the tube must move through the neck of the tube and thus through the electron gun situated in the tube neck. The beam passageways through the gun unavoidably act as high velocity gas conduits as the tube is evacuated. These high velocity gas conduits create a violent flow of gas over the cathodes while the tube is being evacuated. It has been observed that this violent flow of gas over the cathode causes erosion of the electron emissive coating on the cathode (especially the coating of the "green" cathode in an electron gun for a color television pictue tube) which may necessitate rejection of a tube or which may result in degraded performance and/or reliability of a tube.
It is common practice in the manufacture of television pictue tubes to control the humidity during evacuation of the tube within a narrow dew point window. Typically the dew point is controlled between 40.degree. F to 50.degree. F. If the humidity is too high the cathode coating is eroded by particles of moisture during evacuation. It is believed that the moisture particles actually freeze during pump down. This, coupled with the high velocity at the initiation of evacuation and the violent flow of the air through the electron gun in the neck of the tube results in serious erosion of the cathode coating. The narrow dew point window has always presented serious problems in the manufacture of color television picture tubes. The conventional factory process must be constantly and closely monitored and the dew point window shifted with the seasons of the year. The present process is so difficult as to be barely workable. Suppression of the cathode erosion would allow the dew point window to be opened up, and thus allow the same process to be used year around.
Copending application Ser. No. 782, 140 filed Mar. 28, 1977 method of preventing cathode coating erosion wherein gas is pumped slowly from the tube so that the flow of gas through the electron gun does not occur at such a high rate as to cause erosion of the cathode coatings. This method has several drawbacks: (1) an undesirably long time is needed to evacuate the tube, and (2) also the method is not totally reliable.
This invention has general applicability and may be applied to electron gun assemblies in color television tubes as well as to electron guns in black and white tubes. The invention is known to have applicability to a televison picture tube having a narrow neck utilizing either a standard type electron gun or a unique type of electron gun disclosed in U.S. Pat. No. 3,995,194.